1. Field of the Invention
The present invention relates to spread spectrum communication system and to an apparatus for communication utilizing this system. More specifically, the present invention relates to spread spectrum communication system using direct sequence and/or frequency hopping, and to an apparatus for communication utilizing this system.
2. Description of the Background Art
Communication using narrow band modulation system (such as AM, FM, BPSK) has been conventionally used in the field of data communication. In such a system, demodulation at the receiver can be carried out by a relatively small circuitry. However, such a system is weak against multipath fading and narrow band noise.
By contrast, in spread spectrum communication system, data spectrum is spread by a PN code at the transmitter side, while the PN code and the data are synchronized on the receiver side, so that the influence of multipath fading and narrow band noise can be reduced, which system has attracting increasing attention as a promising technique.
The method of spread spectrum communication includes direct sequence, frequency hopping, time hopping and a hybrid combining two or more of these. Direct sequence spreads the spectrum by multiplying data and the PN code having a chip rate considerably higher than data rate, of which circuitry can be implemented relatively easily as compared with those used in other methods. Use of different PN codes allows multiple access in the same band. Such multiple access is called CDMA (Code Division Multiple Access) or SSMA (Spread Spectrum Multiple Access).
FIGS. 1 and 2 are schematic block diagrams of the spread spectrum communication system using direct sequence. FIG. 1 shows the transmitter side and FIG. 2 shows the receiver side.
On the transmitter side, referring to FIG. 1, information 1 represented as a(t) is modulated at an information modulating block 2 to be turned to a signal b(t), which in turn is multiplied by a PN code represented as c(t) generated in a PN code generator 4, at a spreading block (multiplier) 3. PN code generator 4 is driven by clocks from a reference clock oscillator 5. The chip rate of the PN code c(t) is much higher than the data rate of data a(t), and therefore spectrum band of the multiplied output signal s(t) is spread as compared with b(t). The spread multiplied output signal s(t) is converted to RF by a frequency converting block 6, amplified by a power amplifier block 7 and is transmitted through an antenna 8.
On the receiver side, the signal received by an antenna 9 is amplified by an RF amplifier block 10 and is converted to an intermediate frequency at a frequency converting block 11. The signal s(t) which has been converted to the intermediate frequency is multiplied by a PN code c(t) having the same sequence as the code c(t) generated at the transmitter side, in a PN code generator 13. The PN code generated by PN code generator 13 must be synchronized in time with the PN code included in the received signal provided as an input to a despreading block (multiplier) 12. For this purpose, a time discrimination control circuit 14 having a loop structure is prepared, which constitutes, together with the PN code generator 13, a synchronizing block S. With the PN code removed at the despreading block 12, the output b(t) from the despreading block 12 is returned to the narrow band signal modulated only by the data, which signal is passed through an information demodulating block 15 to provide information 16 as represented by a(t).
Since synchronization in time is provided at despreading block 12 on the receiver side, the influence of the multipath fading which comes delayed in time can be reduced. Since the received signal is multiplied by the PN code generated by PN code generator 13, the narrow band noise input to the receiving antenna can be spread, and therefore the influence thereof can be reduced.
As described above, spectrum spreading enables communication in wider bandwidth which is strong against multipath fading and narrow band noise, and thus more effective communication can be carried out.
Details of the spread spectrum communication system shown in FIGS. 1 and 2 are described in Spread Spectrum Communication System, pp. 10-16, published by Kagaku Gijutsu Shuppansha.
The direct sequence spread spectrum communication system is stronger against multipath fading and narrow band noise as compared with the conventional narrow band communication as described above. However, it requires circuits for spreading and despreading spectrum, and since circuits in the synchronizing block employed therein generally has a loop structure, the circuits inevitably becomes large and completed as compared with the receivers for narrow band communication.
As a method of confirming synchronization of the PN code, the PN code of the received signal is multiplied by the PN code generated in the receiver, and the result is integrated. The spreading is carried out dependent on whether the result of integration is at a certain level. In other words, it takes some time to confirm synchronization, which depends on the time of integration. For example, if synchronization is to be confirmed with the chip shifted by 1/2, the maximum time necessary for confirmation is 2n.times.t(s), where t(s) represents the time (sec) of integration and n represents code length of n chips. Meanwhile, frequency hopping is considered promising as it is especially strong against multipath fading.
FIG. 3 shows a power spectrum, FIG. 3(a) shows frequency characteristic of propagation in a propagation path in a room, FIG. 3(b) shows an example of transmission of narrow band modulated wave having a band B.sub.1, and FIG. 3(c) shows spectrum obtained by frequency hopping. As shown in FIG. 3(a) in a propagation path in a room, there are frequencies of which gain is made stronger and frequencies of which gain is made weaker because of multipaths. Assume that a narrow band modulated wave having the band B.sub.1 shown in FIG. 3(b) is transmitted through the propagation path having such characteristic of propagation. This frequency is exactly in the frequency range of which gain is made weaker, and therefore C/N is degraded, causing significant degradation of bit error rate. In frequency hopping such as shown in FIG. 3(c), the frequencies f.sub.1 to f.sub.2 are divided into several slots each having at least the bandwidth of B.sub.1. Several tens to several hundreds of such slots are prepared and the frequency used is changed several bits by several bits. In this case, even when C/N of some of the slots may be degraded, remaining slots have high C/N, and therefore only a small number of bits may cause an error statistically. Errors continuous over several slots can be corrected by employing a method of error correction strong against burst error or interleave. Consequently, stable communication is ensured even in such a propagation path as shown in FIG. 3(a) which is much varied.
As described above, frequency hopping is strong in propagation paths having multipath phasing. However, the circuitry, especially the circuitry in despreading system, is much complicated and large. Therefore, frequency hopping is not popularly used except in a few special systems.
FIG. 4 is a block diagram of an acquisition circuit used in conventional frequency hopping. The acquisition circuit is described in Spread Spectrum Communication System, by Mitsuteru Yokoyama, Kagaku Gijutsu Shuppansha. This is an acquisition circuit for tracking an initial signal at the start of connection of communication. Referring to FIG. 4, a frequency hopping synthesizer 21 generates a local oscillation signal of local frequency corresponding to the hopping frequency, which local oscillation signal is applied to a power combiner 22 to be combined with a reception signal received at an antenna 23, and is converted to an intermediate frequency signal. A prescribed band component of the intermediate frequency signal is taken out by a bandpass filter (BPF) 24, which is squared by a square law 25 and integrated by an integrator 26, so that the signal energy is detected.
A search control logic 27 is provided for eliminating uncertainty of time domain and frequency domain, and it controls oscillation frequency of a VCO 29 by applying a control signal to a frequency control circuit 28 and controls clock frequency from a clock generating circuit 31 by applying a control signal to a clock control circuit 30. Oscillation output from VCO 29 is applied to frequency hopping synthesizer 21 and clock signal from clock generating circuit 31 is applied to a PN sequence generator 32. PN sequence generator 32 applies a control signal to frequency hopping synthesizer 21 based on the clock signal from clock generating circuit 31.
In the acquisition circuit shown in FIG. 4, hopping frequency is determined corresponding to the pattern of the PN sequence. For this purpose, a control signal is applied to frequency hopping synthesizer 21 from PN sequence generator 32, and when synchronization can not be established and timings do not match, phase of the PN sequence is shifted 1/2 chip by 1/2 chip, switching of the hopping frequency is made faster and time domain is searched. If the frequency is deviated, an main oscillator of frequency hopping synthesizer 21 is offset to search the frequency domain.
FIG. 5 is a block diagram of a tracking circuit. The tracking circuit is provided for keeping synchronization after the communication is initially tracked by the acquisition circuit shown in FIG. 4. The tracking circuit includes a frequency correlating network 35 in which an advanced local oscillation signal 1.sub.E (t) than the received signal r(t) and a retarded local oscillation signal 1.sub.L (t) are applied from a frequency hopping synthesizer 43 to power combiners 36 and 37, and the received signal r(t) has its frequency converted. The signal having the frequency converted in this manner have their bands restricted by bandpass filters 38 and 40, squared in square laws 39 and 41, respectively, and these signals are added by an adder 42 to be applied to a loop filter 44. Loop filter 44 removes high harmonics having the period of the hopping frequency as fundamental component from the received signal, the resulting output is applied to a VCC circuit 45, and a control signal is applied to an FH sequence generator 46. FH sequence generator 46 applies a control signal to frequency hopping synthesizer 43 based on the control signal from VCC circuit 45. The tracking circuit shown in FIG. 5 is described in the aforementioned article together with the above described acquisition circuit, and therefore detailed description is not given.
As described above, in the conventional spread spectrum communication system, the acquisition circuit shown in FIG. 4 and the tracking circuit shown in FIG. 5 are necessary, which requires large scale circuitry and makes it difficult to change the frequency at high speed. Although it is strong against multipath fading and suitable for use in a room, synchronization is lost when the communication is disconnected, and in that case, it is necessary to re-track from the start. This takes a long period of time during which the signal can not be demodulated.
Further, for despreading in the spread spectrum communication system, there are active methods of correlation such as sliding correlation and passive methods such as those using a matched filter or a correlator. In the active methods of correlation, synchronization is tracked, and thereafter synchronization is maintained by using a tracking loop such as a DLL loop.
However, in this method, the chip of the code is shifted little by little to find a timing at which codes coincide with each other for synchronization tracking, and therefore it takes long to track synchronization. Therefore, though it is suitable for communication with a fixed propagation path, it could not be applied for communication, in which propagation path changes frequently (as in residence or room), since it takes time to establish synchronization again once synchronization is lost.
FIG. 6 is a block diagram of a demodulator employing a positive despreading system used in such communication. Referring to FIG. 6, input intermediate frequency (IF) signal 1 is applied to a multiplying circuit 50 and has its frequency converted by I and Q components of a local signal generated from a local oscillator 61 and turned into baseband I component 51 and Q component 52. These two inputs are applied to an I channel correlator 53 and a Q channel correlator 54, respectively, and these are corelated by these correlators. The correlated output 55 is input to a data demodulating circuit 56, and data 57 is provided as an output.
Meanwhile, correlated outputs 55 and 58 from I and Q channels are applied to a loop control circuit 59 in which a control voltage for local oscillation signal generator 61 is determined, and the local oscillation signal generator 61 is controlled such that the local oscillation signal and the intermediate frequency carrier wave have their phases synchronized. The example shown in FIG. 6 is similar to a Costas loop.
FIG. 7 shows an output waveform from the correlator 53 shown in FIG. 6. In the demodulating circuit shown in FIG. 6, a correlated output is generated when the PN codes are matched perfectly, and at other portions, the output waveform assumes approximately 0. When the PN code has 127 chips, for example, the correlated output is provided for the time 1/127, and as the loop control circuit 59 provides a loop by using a phase difference read from this 1/127 time, a circuitry somewhat different from the general analog type Costas (PLL) loop is necessary. In the data demodulating circuit 56, determination as to whether the data is 1 or 0 is carried out based on this pulse-like signal, and data is obtained by recovering clocks.
As described above, in the passive type circuitry, demodulation can be carried out in the similar manner as in the general narrow band digital demodulation. However, there are following problems.
More specifically, since the receiver includes a carrier recovery circuit, it takes time to until the synchronization of the carrier is established. Though the necessary time is much shorter than that in the active type, data can not be demodulated for this time necessary for synchronization when carrier synchronization is frequently lost because of the frequent change of paths.
FIG. 8 is a diagram of a signal waveform showing correlator output when there is much multipath fading. As shown in the figure, there are several correlator outputs corresponding to the difference in time of propagation of the multiple paths. In such a case, it is difficult to determine which of the outputs is to be used for obtaining data. In addition, if wave detection is to be done by using total power, it is necessary to provide a window on the time axis for integration. However, in such a case as shown in the figure, it is difficult to determine how and when a window is to be provided, since the optimal values differ path by path.
Further, the output data has two stable points, that is 0 and .pi. because of the carrier phase, and since data is inverted, it is necessary to employ differential coding or to carry out discrimination using a preamble signal of the data.
For multiple access by a large number of users, different PN codes are used for discrimination. However, the m sequence of 127 chips, for example, has only 18 codes, and therefore it is not available for larger number of users.